A microgrid is a localized grouping of electricity generation, energy storage, and loads that normally operates connected to a traditional centralized grid (macrogrid) via a point of common coupling (PCC). This single point of common coupling with the macrogrid can be disconnected, islanding the microgrid. Microgrids are part of a structure aiming at producing electrical power locally from many small energy sources, DGs. In a microgrid, a DG is connected via a converter which controls the output of the DG, i.e. the current injected into the microgrid.
A microgrid (in grid connected mode, i.e. connected to the macrogrid) supplies the optimized or maximum power outputs from the connected DG sites and the rest of the power is supplied by the macrogrid. The microgrid is connected to the macrogrid at a PCC through a controllable switch. This grid connection is lost during grid fault and the microgrid is islanded.
During islanding, there is a risk of imbalance in the microgrid due to the loss of power import from grid as well as loss of voltage control by the grid. For voltage control it is required to change control mode of the DGs. The power balancing is solved by fast storage action and immediate load shedding schemes.
The change in control mode of the DGs is from current source to voltage source operation and is initiated to create the voltage and frequency reference for the microgrid without presence of the macrogrid.
This switchover from one mode to another is initiated by island detection and thus the set point tracking of the converters depend on the island detection time, mode change signal and settling time of the primary control loop.
One of the major drawback in sudden loss of grid connection following an abrupt change in control mode may lead to high rate of discharge (or charge) in storage which may cause tripping leading to a major power imbalance and stability problems.
CN 101 902 146 is a regular Chinese application which appears to disclose an inverter in a microgrid, which inverter is controlled as a current source when connected to an electrical network and as a voltage source in island operation.